Until now there has been no completely satisfying solution to the problem of comfortably conforming hard enclosures to the shapes of the portions of the human body enclosed therein. Similarly, such conformations within rigid enclosures make it difficult to simultaneously cushion the supported portion of the human body, as is required when adapting outer shells of ski boots and the like, which consist of rigid cured or thermoset synthetic-resin polymers conforming them to the contours of the enclosed feet while cushioning the feet against shocks.
Similar problems are encountered in connection with shoes, trusses, corsets, girdles, protective helmets and junctures between prosthetics and prosthetic appliances and the human body. Particularly of interest are the cushions required for the resilient connection between artificial limbs and the portions of the body to which they are applied.
It has been known to adapt ski boots and shoes to the foot by pouring into the cavity between the foot and the shoe a silicone rubber and vulcanizing or curing the silicone rubber in the presence of the foot (U.S. Pat. No. 3,325,919). Cushions produced in this manner have the drawback that once the silicone rubber has been vulcanized, the resultant hardened rubber is inalterable in shape and is quite heavy.
It also has been proposed to fill the cavities between rigid articles such as ski boots and the feet, with packets or envelopes filled with hard microspheres of a thermosetting material which are covered with a lubricating agent (U.S. Pat. No. 3,407,406, German Published patent applications DT-OS 1,485,772 and DT-OS 2,211,718). Such microsphere packets essentially are filled with incompressible phenolic resin hollow balls. Such cushions will assume a shape conforming to the shape of the cushioned body portions but have been found not to provide sufficient yielding in their support because such cushions cannot be reversibly or elastically compressed as is, for instance, the case with soft synthetic foamed material. Thus it becomes necessary to additionally interpose a soft foamed material between the packets of microspheres and the body part.
It has further been proposed to fill the cavities between the rigid enclosure and the body part with cushions, packets and similar enclosures of a pasty and semiliquid thixotropic substance having a high viscosity such as a polyisobutylene containing mixed therein a large proportion of solid filler material (German Published patent application DT-OS 1,685,307). However, the cushioning effect of such enclosures is insufficient because the solid filling material within such cushions cannot be reversibly compressed when subjected to dynamic pressure. Thus, when such materials are used, an additional cushioning layer such as a foam, possessing sufficient compressibility must similarly be included or interposed.
It has also been proposed to utilize foamed-in-situ polyurethane foams, thereby providing the required adaptation of the filling material to the existing cavity between the rigid enclosure and the body part (German patent DT-PS 901,471). Such proposals suffer from the drawback that the individual adaptation, by definition, must be effected in situ and is objectionable, both because of the toxicity of isocyanates used in the manufacture of the foams and the reaction heat liberated during the foaming process.
Further, misadaptations often occur which, because of the irreversible nature of the reaction, cannot be readily corrected.
Another drawback to the use of foamed-in-place materials is the low resistance of polyurethane foams to sweat and its hydrolysis by moisture. This low resistance within a short period of use time destroys the elasticity of the polyurethane foam and leads to its degradation. It thus can no longer serve its intended purpose.
It has also been proposed to fill packets or cushions with shredded particles of soft elastic foams. These particles in the shape of leaves or slices have great specific surface (German published patent application DT-OS 1,010,825). The desired permanent individual adaptation by this procedure to the shapes of the body cannot be effected for any extended period of use, because the foamed material is neither thermoplastic nor thermoelastic, and the loose particles within the packets or cushions tend to shift position during dynamic use and lose contour conformation. Furthermore, as is the nature of such foams, the cushions are subject to deterioration by perspiration and moisture hydrolysis within the packets as a result of contact with the body.
It has also been proposed to fill cushions or packets with extruded fillings in the shape of a rope, cord, tow or string fashioned from foamed thermoplastic or thermosetting material. Such cushions similarly do not effect a permanent adaptation to the body parts and in addition it has been found that the support is insufficiently elastic or resilient as regards support during shock for the body parts.
It has also been suggested to replace the foamed polyurethane with foamed polystyrene in the form of chips, granules or powders for filling cushions and mattresses. Such foamed polystyrene rapidly loses its elasticity after repeated exposure to stress over extended periods of time and due to such fatigue loss of elasticity, it cannot serve for extended use.
It has likewise been suggested to use hollow microspheres and similar hollow particles as filler materials to provide low-density products. These microspheres have been suggested for use as fillers in such substrates as linoleum and floor tiles or in aggregates such as concrete and plaster. As described in U.S. Pat. No. 2,797,201, these are rigid articles and the microspheres therein serve to provide a filling action. This patent describes hollow particles being adhered together to produce solid, cellular-type materials and/or rigid honeycomb core materials for various structural, decorative or special purpose panels. These microspheres, loose or bound together are enclosed within rigid skins such as plywood, metal, plastic laminates and other similar rigid face materials.
It has further been suggested that these materials, due to their hollow structure, be loose-poured into location, packed under pressure and sealed in situ to provide a static fill-type thermal insulation.